The basic principles and techniques for electromagnetic logging for earth formations are well known. For example, induction logging to determine the resistivity (or its inverse, conductivity) of earth formations adjacent a borehole has long been a standard and important technique in the search for and recovery of subterranean petroleum deposits. In brief, a transmitter transmits an electromagnetic signal that passes through formation materials around the borehole and induces a signal in one or more receivers. The amplitude and/or phase of the receiver signals are influenced by the formation resistivity, enabling resistivity measurements to be made. The measured signal characteristics and/or formation properties calculated therefrom are recorded as a function of the tool's depth or position in the borehole, yielding a formation log that can be used by analysts.
Note, however, that the resistivity of a given formation may be isotropic (equal in all directions) or anisotropic (unequal in different directions). Formation resistivity logging is further complicated by the presence of borehole fluids. Drillers employ borehole fluids to cool the drill bit, remove drill cuttings, and to preserve the integrity of the borehole. Some borehole fluids are water-based and tend to be fairly conductive, while other borehole fluids are oil-based and tend to be highly resistive. While various existing resistivity logging tools perform well in water-based fluids, the options for resistivity logging in oil-based fluids are much more limited.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.